This invention relates to the measurement of acid sulfate levels in an aerosol, such as in ambient air, and in particular to an infrared quantitative measurement method for fine particles such as bisulfate in ammonium bisulfate.
Fine particle sulfates constitute the major source of acid aerosol in the ambient and indoor air. Acid aerosols pose a threat to human health and well-being which has been recognized and which is the basis for recent regulatory control of the precursors for acid aerosol as part of the Clean Air Act Amendments of 1990. However, measuring the existence and levels of acid sulfate in air has proven to be difficult and remains the subject of continuing research which is necessary to establish a scientific basis for regulatory action in protecting persons exposed to acid sulfate in air.
One method for determining particle acidity involves the collection of acid particles on a filter and the water extraction of the particles for analysis of hydrogen ion, ammonium ion, sulfate ion and other ions by, for example, ion selective electrode or ion chromatography. The acidity is often associated with the total concentration of hydrogen ion although the chemical species from which the hydrogen ion is extracted is invariably in question. These methods do not allow speciation of sulfate into ammonium sulfate, ammonium bisulfate and sulfuric acid. This distinction is important because each species of sulfate has different potential health effect. Also, present measurement methods are not fully satisfactory in a number of respects and in meeting present needs as discussed further below. In particular, it is highly desirable that the measurement method sample can be preserved for additional, complementary analyses such as X-ray fluorescence, or wet chemical analyses. Moreover, it is desirable to be able to detect and to provide a numerical value related to acid sulfate in the particle sample at the relatively low concentration levels routinely present in the ambient atmosphere.